Vertical-cavity surface-emitting lasers (VCSELs) are of great interest because of their good performance and wide-ranging applicability. Low fabrication costs requires a high production yield, and it is therefore a requirement that the fabrication method used for their production is strictly controllable. A high yield is obtained only when each and every processing step of the fabrication method is under control.
Fabrication of VCSELs containing a lateral layer with an etched micro/nano-structured mode selective layer for controlling the electromagnetic modes in the VCSEL include extra fabrication steps. The process may involve steps such a resist deposition, resist exposure, resist development, etching or material deposition, and resist removal. Obtaining a high yield in fabrication of micro/nano-structured VCSELs requires fine control of all these steps.
The VCSEL performance depends critically on the etch depth when fabricating the micro/nano-structured mode selective element. The etch depth is typically determined by the etch rate so that the total etch time determines the etch depth. Unfortunately, the etch rate depends on many parameters; for wet chemistry etching e.g. temperature, chemicals, concentration; for dry chemistry etching e.g. gas-flow, gas-pressure, used gasses amount of reactive and non-reactive gasses, and gas mixture.
In “Electrically injected single-defect photonic bandgap surface-emitting laser at room temperature”, Electronics Letters, vol. 36, no. 18 (31 Aug. 2000), authors W. D. Zhou et al. disclose a VCSEL structure comprising a bottom mirror, a top mirror, and a gain region sandwiched in between the two mirrors. Both mirrors are III-V semiconductor based. Deep holes are etched from the top of the top mirror, through the gain region, and into the bottom mirror. The holes are furthermore laid out in a regular array, with a defect in the center of the array. The etch depth is determined by estimating an etch rate and timing the etch. The problem of a well controlled etch depth is not addressed in the paper.
In U.S. Pat. No. 6,683,898, a VCSEL structure having a bottom mirror, a top mirror, and a gain region sandwiched in between the two is disclosed. A photonic crystal region is formed to prevent higher-order transverse modes in the VCSEL. The fabrication of the disclosed structures is complicated by re-growth steps and/or etch depths determined by timing the etch processes.
In “True photonic band-gap mode-control VCSEL structure”, ECOC'03 pp. 40-41 (2003), authors F. Romstad et al. demonstrate how the wavelength can be well controlled by shallow etching (less than 100 nm) in a VCSEL. The shallow etching is done in a partial semiconductor top-mirror and the necessary top-mirror reflectivity needed to achieve lasing is obtained by depositing a dielectric top-mirror on top of the locally etched partial semiconductor top-mirror.
In “Single-mode photonic bandgap VCSELs”, ECOC'04, Proceedings vol. 3, pp. 596-597 (2004), authors S. Bischoff et al. demonstrate a VCSEL relying on lateral mode confinement by the Photonic BandGap (PBG) effect. The PBG effect was implemented by shallow etching of rods in a partial semiconductor top-mirror. The top-mirror reflectivity needed to achieve lasing is obtained by depositing a dielectric top-mirror on top of the locally etched partial semiconductor top-mirror.
International application PCT/DK2005/000759 discloses a technique for lateral mode control in VCSELs, capable of providing large-aperture single-mode high-power VCSELs. The invention gives an improved VCSEL design by providing basic structural details allowing large-aperture single-mode high-power operation. A number of structures described therein are characterized in that they comprise a central light aperture region, which provides a long photon lifetime and overlaps with the active region. They also comprise a mode-shaping region formed so as to provide a photon lifetime shorter than that in the light aperture region. Furthermore, they comprise a mode confinement region designed to provide lateral confinement of modes to the mode-shaping region and the light aperture region. The dimensions of the regions are selected to engineer an efficiency of laser action in each transverse electromagnetic mode of a cavity in the VCSELs.
The above prior art descriptions of prior art VCSELs are all based on etching of a micro/nano-structured mode selective region in a complete or partial semiconductor top-mirror. A disadvantage of the methods is the need for a very well controlled etch process. The lack of high etch-depth control requires additional process characterization to determine the actual achieved etch depth.
Handling of a device during an etch process can be a significant source of process variation, especially when it comes to wet chemistry etch techniques. The time it takes to start and stop the etch process can contribute to the uncertainty, in particular if the etch rate is relatively high. This adversely affects production yield if not under control.